RNA interference (RNAi) is a posttranscriptional gene-silencing event in which short double-stranded RNA (siRNA) degrades target mRNA. Because of its potent and highly specific gene-silencing effect, RNAi is expected to be used in the treatment of various diseases. Cancer is one of the major targets of RNAi-based therapy, because silencing oncogenes or other genes contributing to tumor progression can be target genes for RNAi The delivery of RNAi effector to target cells is one of the key factors determining therapeutic efficacy, because gene silencing is limited to cells reached by RNAi effectors. Tumor cell lines stably expressing reporter genes were confirmed to be effective in sensitively and quantitatively evaluating RNAi effects in tumor cells in vitro and in vivo. Quantitative analyses of the gene-silencing effect revealed that short-hairpin RNA expressing plasmid DNA (pshRNA) has more durable effects than siRNA. Intratumoral injection of RNAi effectors was effective in suppressing target gene expression in tumor cells, and silencing of beta-catenin or hypoxia-inducible factor-1 alpha (HIF-1 alpha) significantly inhibited tumor growth. RNAi effectors were successfully delivered to tumor cells colonizing the liver through the vascular route. We found that tumor-bearing liver showed elevated HIF-1 alpha expression in the cells, and the silencing of the expression in normal liver cells is also effective in inhibiting metastatic tumor growth. These results indicate the possibility of RNAi-based cancer therapy.